The present invention generally relates to semiconductor devices and more particularly to an improvement of a semiconductor device having a selectively doped heterostructure such as the high electron mobility transistor (HEMT).
HEMT devices are now becoming available in the form of discrete devices or in the form of integrated circuits, similarly to the case of other, conventional semiconductor devices. As is well known, a typical HEMT device has a structure of the field effect transistor wherein a two-dimensional electron gas is formed at a heterojunction interface between an undoped channel layer of GaAs and a doped, carrier supplying layer of AlGaAs, and this two-dimensional electron gas is used as the medium for transporting the carriers at an enormously high speed.
With the spreading of use of the HEMT device in various fields, a number of specific problems are encountered. For example, there is a problem called "real space transfer" in which the electrons transported through the channel layer from the source region to the drain region of the HEMT device is accelerated excessively by the large electric field between the source and the drain regions. When such an acceleration occurs, the electrons invade into the carrier supplying layer, overriding the potential barrier formed at the heterojunction interface as a result of the increased energy. This phenomenon occurs most significantly in the region between the gate and the drain of the HEMT device in which the acceleration of the electrons becomes the maximum. Once the electrons enter into the carrier supplying layer, the electrons neutralize the positive space charges which are formed as a result of the formation of the two-dimensional electron gas. In other words, such an invasion of the electrons into the carrier supplying layer causes a vanishing of the two-dimensional electron gas which in turn causes a large voltage drop across the gate and the drain of the HEMT device. Thereby, the operational characteristic of the HEMT device is significantly deteriorated. In the conventional HEMT devices, the problem of the foregoing real space transfer of the electrons is difficult to eliminate. This problem becomes particularly serious when the gate length of the HEMT device is reduced for high speed operation and high integration density. In the description hereinafter, the problem of the real space transfer will be examined briefly.
FIG. 1 shows a typical example of the band diagram at 77K of the conventional selectively doped heterostructure wherein an n-type AlGaAs carrier supplying layer L.sub.ES having a composition of Al.sub.0.3 Ga.sub.0.7 As is grown on an undoped GaAs active layer L.sub.AV. In the drawing, the energy levels of the .GAMMA. valley, the L valley and the X valley of the conduction band are designated respectively by Ec1, Ec2 and Ec3, wherein FIG. 1 shows the profile of Ec1, Ec2 and Ec3 taken along the thickness of the foregoing heterostructure of the layers L.sub.AV and L.sub.ES. Further, the energy discontinuities appearing at the heterojunction interface in the profile of the .GAMMA. valley, the L valley and the X valley are designated respectively by .DELTA.E.sub.D1, .DELTA.E.sub.D2 and .DELTA.E.sub.D3. W.sub.T designates a "window of transition" to be described later.
In the foregoing selectively doped heterostructure, the energy discontinuities .DELTA.E.sub.D1, .DELTA.E.sub.D2 and .DELTA.E.sub.D3 assume respectively the values of 0.21 eV, 0.04 eV and -0.07 eV. In this structure, although the energy discontinuity .DELTA.E.sub.D1 takes a value sufficiently large to form the two-dimensional electron gas at the heterojunction interface, there is a tendency that the electrons, when excited to the L valley upon the acceleration in the channel layer L.sub.AV, invade into the carrier supplying layer L.sub.ES, overriding the small energy discontinuity .DELTA.E.sub.D2 at the heterojunction interface. In other words, there occurs the problem of the real space transfer. Similarly, the electrons excited to the X valley can easily cause the real space transfer to the carrier supplying layer L.sub.ES, as the energy level of the L or X valley of the conduction band of the carrier supplying layer L.sub.ES is lower than energy level of the X valley of the conduction band of the channel layer L.sub.AV by 0.07 eV.
A similar real space transfer occurs also between the L valley of the channel layer L.sub.AV and the .GAMMA. valley of the carrier supplying layer L.sub.ES. Most importantly, there is a tendency that the electrons, causing the transition from the .GAMMA. valley to the L valley in the channel layer L.sub.AV in response to the intervalley scattering, cause the real space transfer to the .GAMMA. valley of the carrier supplying layer L.sub.ES through the window of transition W.sub.T before they complete the transition to the L valley in the channel layer L.sub.AV . This window W.sub.T is formed as a discontinuity between the L valley in the channel layer L.sub.AV and the .GAMMA. valley in the carrier supplying layer L.sub.ES and has an energy difference of 0.05 eV. As the proportion of the electrons causing the transition to the energy level of the window W.sub.T in the channel layer L.sub.AV is much larger than the proportion of the electrons causing the transition to the L valley or X valley, the effect of the real space transfer of the electrons through the window W.sub.T is not negligible at all particularly when there is a large acceleration of electrons between the source and drain of the HEMT device associated with the decreased gate length.
Although there are proposed various material systems for the selectively doped structure of HEMT devices, the problem of the window of transition W.sub.T appearing in the band structure and the real space transfer of electrons occurring through the window W.sub.T has never been considered hitherto.
FIG. 2 shows the band diagram at 77K of a selectively doped heterostructure comprising an n-type carrier supplying layer L.sub.ES having a composition of Ga.sub.0.516 In.sub.0.484 P and the channel layer L.sub.AV having a composition of GaAs. As can be seen clearly, there is a discontinuity between the L valley in the channel layer and the .GAMMA. valley in the carrier supplying layer, forming the window of transition W.sub.T similarly to the case of FIG. 1. In this material system, the parameter .DELTA.E.sub.D1 assumes a value of 0.22 eV, the parameter .DELTA.E.sub.D2 assumes a value of 0.34 eV, and the parameter .DELTA.E.sub.D3 assumes a value of 0.19 eV. Thereby, the window W.sub.T is formed in response to the energy discontinuity of 0.04 eV appearing between the L valley in the channel layer L.sub.AV and the .GAMMA. valley in the carrier supplying layer L.sub.ES at the heterojunction interface. Thus, the HEMT device using the selectively doped heterostructure of FIG. 2 has the problem of the real space transfer of electrons similarly to the case of FIG. 1.
FIG. 3 shows another example of the band diagram of the selectively doped heterostructure wherein an n-type carrier supplying layer L.sub.ES having a composition of Al.sub.0.52 In.sub.0.48 As is provided on an undoped channel layer L.sub.AV of Ga.sub.0.53 In.sub.0.47 As. In this structure, there appear energy discontinuities .DELTA.E.sub.D1, .DELTA.E.sub.D2 and .DELTA.E.sub.D3 such that .DELTA.E.sub.D1 =0.5 eV, .DELTA..sub.ED2 =0.13 eV and .DELTA..sub.ED3 =-0.03 eV. In this case, a large potential barrier appears in the .GAMMA. valley in association with the large, positive energy discontinuity .DELTA..sub.ED1. In the profile of the L valley, on the other hand, it will be understood that the energy level of the carrier supplying layer L.sub.ES at the heterojunction interface is higher only by 0.13 eV than the corresponding energy level of the channel layer L.sub.AV. Furthermore, there appears the window of transition W.sub.T, as the .GAMMA. valley of the carrier supplying layer at the heterojunction interface is lower than the L valley of the channel layer by about 0.03 eV. Thus, the electrons are spatially transferred from the channel layer L.sub.AV to the carrier supplying layer L.sub.ES passing through the window of transition W.sub.T. It should be noted that the electrons have a very high mobility in the .GAMMA. valley of the channel layer L.sub.AV and are easily accelerated to an energy level sufficient to override the potential barrier .DELTA.E.sub.D1. Further, it will be seen that there appears another window W.sub.T ' between the L valley and the X valley in this structure and the electrons will cause the real space transfer through the window W.sub.T ', when they are excited in response to the acceleration.